1. Field of Invention
The present invention relates to the technical field of power electronics. More particularly, the present invention relates to a control circuit, control method used in a PFC circuit and the power source system including the control circuit.
2. Description of Related Art
Currently, in order to reduce the serious harmonic pollution to a power grid caused by frequently-used power electronic devices, generally there is a need to introduce a power factor correction (PFC) circuit, so as to make the input current harmonic meet the requirements of a predetermined harmonic through the PFC circuit. Additionally, the development trend of the PFC circuit is towards the direction of high efficiency and high power density, just as most of the power source products.
Taking a bridgeless PFC circuit topology as an example, the circuit has many advantages, such as low conduction losses, low common-mode interfere and high utilization ratio of components. For example, the bridgeless PFC circuit includes a first bridge arm and a second bridge arm connected with each other in parallel. The first bridge arm is formed by a first MOSFET and a second MOSFET. The second bridge arm is formed by a first diode D1 and a second diode D2.
When the second MOSFET is off and the first MOSFET is on in the first bridge arm, the inductor releases energy through the first MOSFET and a fourth MOSFET. Accordingly, the inductor current decreases. Afterwards, the inductor current decreases to zero at a certain moment and after the moment the direction of the current is reversed. When the first MOSFET is off, a voltage (VDS) across two ends of the drain electrode and the source electrode of the second MOSFET starts to decrease. If the second MOSFET is controlled to be turned on when the voltage decreases to zero, it is realized to turn on the second MOSFET under a zero voltage, so as to reduce the switch loss. However, it is a subject for related technicians to solve which control mechanism is used to realize the above-mentioned turn-on under the zero voltage so as to reduce the switch loss of the circuit. Additionally, it is a task for designers to solve how the zero-crossing point of the inductor current is automatically detected in a simple and effective way.